[#2] Benchmark message serialization latency for v0.2.0 release

[orecham]

Notes for Reviewer

1. A number of tweaks to ensure no regression in communication latency
2. Update benchmarking scripts to:
   1. Include a flag to switch between self-contained and non-self-contained message types
   2. Plot different benchmarking results (identified by directory) on a single figure
3. Update the README to highlight the impact serialization of non-self-contained messages has on communication latency

Pre-Review Checklist for the PR Author

• Add sensible notes for the reviewer
• PR title is short, expressive and meaningful
• Relevant issues are linked in the References section
• Every source code file has a copyright header with SPDX-License-Identifier: Apache-2.0 OR MIT
• Branch follows the naming format (rmw-iox2-123-implement-graph-api)
• Commit messages have the issue ID ([#123] Add posix ipc example)
• Tests exist for new behavior
• Changelog updated in the unreleased section including API breaking changes
• Assign PR to reviewer

Checklist for the PR Reviewer

• Commits are properly organized and messages are according to the guideline
• Unit tests have been written for new behavior
• Public API is documented
• PR title describes the changes

Post-review Checklist for the PR Author

• All open points are addressed and tracked via issues

References

Closes #

[elBoberido]

nitpick: the comment mentions current but the variable is called next. What do you think about also using current for the variable name?